Host: The Japanese Association of Mineralogists, Petrologists and Economic Geologists
The phase relations in the compositional range from 35 to 55 at. % S including Co4S3, Co9S8 and Co1-xS in the Co-S system were investigated at temperatures from 700 to 1200°C using an evacuated silica glass-tube method. The products were examined by means of microscopy, high-temperature X-ray diffraction, DTA and EPMA. In this work the phases appeared are αCo, high-temperature solid solution (Co4+xS3−Co9-yS8), cobalt pentlandite(Co9S8), Co monosulfide(Co1-xS), cattierite(CoS2) and liquid.
Cobalt pentlandite is stable below 831°C, and transforms into a high form at this temperature. This inversion is reversible. The high form makes a solid solution(SS) with Co4S3 above 831°C. This SS is provisionally named as high-temperature SS. It appears as a leaf-like form with the compositional range from 40.8(876°C) to 49.2(829°C) at. % S in maximum including Co4S3 and Co9S8 at temperatures from 806 to 930°C, and coexists with liquid, αCo, cobalt pentlandite and Co monosulfide. High-temperature SS breaks down into liquid and Co monosulfide at 930C and 46.5 at. % S(peritectic point) as its upper stability limit. It also breaks down into cobalt pentlandite and Co monosulfide at 829°C and 49.2 at. % S(S-rich eutectoid) and αCo and cobalt pentlandite at 806C and 42.9(∼Co4S3) at.% S(Co-rich eutectoid) as its lower stability limit. The SS is unquenchable. Cobalt pentlandite has a very limited SS from 46.4 to 47.1(∼Co9S8) at. % S at 800°C, but converges to stoichiometric Co9S8 at 700°C or below. It appears due to exsolutions from high-temperature SS and Co monosulfide and breakdown of its SS at two eutectoids besides inversion of the SS with Co9S8. It is quenchable. Co monosulfide forms a limited SS with the compositions from 50.0 to 53.0 at. % S at 1000C and from 51.7 to 53.8 at. % S at 700°C, and coexists with liquid, high-temperature SS, cobalt pentlandite and cattierite. It melts congruently at 1176°C and 51.8 at. % S.
High-temperature SS belongs to a primitive cubic cell and its cell edges with Co4S3 and Co9S8 are a = 5.178 and 5.171A, respectively, at 850°C, and a=5.183 and 5.175A, respectively, at 875°C. The value of the latter corresponds to about half of cobalt pentlandite(low form) cell edge a = 9.9287A at room temperature. This inversion is thought to be an order-disorder type from supercell(low form) to subcell and is analogous to high-form pentlandite.
